The present intention relates to medical apparatus for monitoring physiological functions; and more particularly to such apparatus which detect a patient's blood pulse rate and period from circulatory data.
Pulse oximetry is a non-invasive technique which is useful for measuring arterial oxygen saturation and pulse rate. In pulse oximetry, light is passed through a portion of a patient's body, such as a finger, which contains arterial blood flow. An optical sensor detects the light which has passed through the body and the variations in the detected light at various wavelengths then are used to determine arterial oxygen saturation and pulse rates. For example, U.S. Pat. No. 5,437,275 illustrates an optical finger probe for a pulse oximeter which produces an electrical signal indicative of the arterial blood flow. Pulse rates also can be detected by conventional invasive and non-invasive blood pressure monitoring equipment. Sensors have been developed to produce electrical signals indicative of these other physiological functions.
All of these types of equipment produce and analyze an electrical signal having a periodic waveform that corresponds to the physiological function being monitored. Although the signal does not fluctuate significantly between consecutive cycles, the amplitude, frequency and shape of the periodic waveform can vary significantly among different persons depending upon their physical robustness and health. The frequency, amplitude and shape of the signal representing the physiological function of the same person also can vary greatly depending upon the level of physical activity and health at any moment in time.
One of the difficulties encountered in extracting information about the particular physiological function from the corresponding electrical signal is the effect that noise has on the signal. Noise can come from a number of sources including electrical noise produced in the sensors and amplifiers that process the electrical signal, as well as physical effects such as movement of the patient during the examination. For example, derivation of pulse rate from the periodic signal produced by the probe of a pulse oximeter can be affected greatly by movement of the finger to which the probe is attached. Such movement may produce a spurious electrical pulse in the signal from the probe, which may be misinterpreted as being produced by arterial blood flow and thus materially affect the pulse rate determination.